Airship launch from a cargo airship with a payload return vehicle

ABSTRACT

A method for launch of an airship includes connecting a cargo airship to a second airship that is not positively buoyant at the launch site, launching the cargo airship, transferring lifting gas from the cargo airship to the second airship where said lifting gas is carried by the cargo airship while aloft; and releasing the second airship from the cargo airship. A releasable payload return vehicle, wherein the payload return vehicle generates aerodynamic forces while it is mated to the cargo airship.

RELATED DOCUMENTS

The present application is a continuation-in-part of U.S. application Ser. No. 15/581,681 filed Apr. 28, 2017, entitled “Airship Launch from a Cargo Airship” which is a divisional and claims the benefit under 35 U.S.C. § 120 of U.S. patent application Ser. No. 14/506,134 filed Oct. 3, 2014 to Stephen Heppe, entitled “Airship Launch from a Cargo Airship”, now issued as U.S. Pat. No. 9,669,917 which is a continuation-in-part of U.S. application Ser. No. 13/347,371 filed Jan. 10, 2012 to Stephen Heppe, and entitled “Airship Launch from a Cargo Airship”, now issued as U.S. Pat. No. 9,522,733 which is a continuation-in-part of U.S. application Ser. No. 13/159,215 filed Jun. 13, 2011, now issued as U.S. Pat. No. 8,864,063 on Oct. 21, 2014, to Stephen B. Heppe, and entitled “Tethered Airships.” These applications are herein incorporated by reference in their entireties.

BACKGROUND

High-altitude airships can be used as platforms for a variety of missions, including weather and astronomical observations. High-altitude airships are designed to be lightweight and hold large volumes of lifting gas to provide the desired amount of buoyancy in the upper atmosphere. A stratospheric balloon or airship is generally designed with a light-weight hull to contain lifting gas while minimizing overall airship mass. For example, airships intended for operation in the upper stratosphere may have a hull with a thickness that is less than 50 μm and an areal density less than 100 g/m² of effective hull surface area, with a surface area on the order of tens of thousands of square meters.

The large surface area and thin hull can make the airship vulnerable to damage, particularly during launch. To launch an airship, a suitable launch site is selected and a launch window is selected when little or no wind is anticipated. The launch method restricts the amount of slack balloon material that is subject to wind drag or “sail” effect during the launch. The launch site also includes a large open area where the balloon can be laid out and inflated without risk of the fragile hull coming into undesirable contact with external objects. Here, the bulk of the balloon is laid out lengthwise on a suitable launch surface. Very large balloons (20-40 million cubic feet displacement; 500,000 to 1,000,000 cubic meters displacement) can use 800 ft (240 meters) or more of layout space. The top portion of the balloon is placed under a roller arm of a launch vehicle. This launch vehicle confines the lifting gas to the top portion of the balloon during inflation. At the completion of inflation, the launch arm is released and the balloon rises vertically over a payload release vehicle. This payload release vehicle includes a crane that suspends the payload. The payload release vehicle can be driven downwind to minimize the wind effects on the hull. Even with these precautions, these launch techniques can only be used in calm or near calm winds and still result in a significant risk of the hull and/or payloads being damaged. Further, these operational constraints severely limit the locations and times that a high-altitude balloon can be launched.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings illustrate various examples of the principles described herein and are a part of the specification. The illustrated examples are merely examples and do not limit the scope of the claims.

FIGS. 1A and 1B show cargo airships with payload doors that can be opened when launching a high-altitude airship, according to one example of principles described herein.

FIG. 1C is a side view of a cargo airship that has been partially cut away to show storage of a high-altitude airship and launching apparatus, according to one example of principles described herein.

FIG. 1D shows a high-altitude airship being launched from a cargo airship, according to one example of principles described herein.

FIG. 1E is a side view of a high-altitude airship and payload at altitude, according to one example of principles described herein.

FIGS. 2A and 2B are diagrams showing high-altitude airship with an inflation tube, according to one example of principles described herein.

FIG. 3A-3C show a storage concept for a high-altitude airship carried within a payload bay of a cargo airship, according to one example of principles described herein.

FIG. 4 is a diagram deployment of a high-altitude airship that has been folded in a payload bay of a cargo airship, according to one example of principles described herein.

FIG. 5 is a flow chart of a method for deploying a high-altitude airship from a cargo airship, according to one example of principles described herein.

FIGS. 6A-6B show a flow chart of a method for deploying a high-altitude airship from a cargo airship, according to one example of principles described herein.

FIGS. 7A-7B each depict front and side views of a releasable mass, including an unmanned aerial vehicle attached to a cargo airship, according to an example of the principles described herein.

FIG. 8 depicts a front and side view showing features of a high altitude airship adapted to operate over an extended range of altitudes according to an example of the principles described herein.

Throughout the drawings, identical reference numbers designate similar, but not necessarily identical, elements.

DETAILED DESCRIPTION

The figures and specification below describe a staged deployment concept that includes a robust low-altitude airship that can carry at least a high-altitude balloon or airship, and optionally a payload return vehicle, as separate payloads. The low-altitude airship can deploy the high-altitude airship at a suitable altitude, and subsequently (for example at end of mission), release a payload return vehicle for payload recovery on the surface of the Earth. In at least one example, the payload return vehicle is a winged heavier-than-air aircraft, such as a glider, that can also be used for aerodynamic control of the airship system prior to its release. The low-altitude airship can be launched (along with its cargo) from a convenient launch site and flown to a different site for the deployment and launch of the high-altitude airship. This enables direct insertion into difficult environments such as polar or mid-oceanic areas.

The robust low-altitude airship can be launched in a greater variety of wind conditions and a greater variety of launch sites (airports), as compared to a fragile high-altitude airship, thereby increasing launch opportunities and reducing certain transportation and logistics costs. Also, by deploying the high-altitude airship at a suitable altitude away from the ground while the low-altitude airship (which may also be called the cargo airship, first-stage airship, or other suitable name) is in drifting flight, airspeed and wind gusts are minimized, thereby easing the launch of the high-altitude airship and minimizing the potential for damage.

The concepts described herein represent a technological alternative to the special-purpose launch site and special-purpose support equipment and personnel used for the launch of high-altitude airships. Additionally, the balloon or high-altitude airship is handled less and risk of damage to the hull is considerably reduced. Additionally, the winged payload return vehicle described herein can provide for reliable payload return (as compared to attempted landing of an entire airship after an extended period aloft) and avoids or minimizes the use of aerodynamic control surfaces on the low-altitude (cargo) airship.

Some or all of the expense of building and operating the low-altitude airship is offset by the avoided expenses associated with some special-purpose launch sites and special-purpose support equipment used for the launch of high-altitude airships. Also, because of the increased operational flexibility of the concepts described herein, costs of launching a high-altitude airship can be amortized over a greater number of launches.

In the following description, for purposes of explanation, numerous specific details are set forth in order to provide a thorough understanding of the present systems and methods. It will be apparent, however, to one skilled in the art that the present apparatus, systems and methods may be practiced without these specific details. Reference in the specification to “an example” or similar language means that a particular feature, structure, or characteristic described in connection with the example is included in at least that one example, but not necessarily in other examples. In some cases, the components shown in the figures may not be drawn to scale. Further, the relative scale of the components in a given figure may be varied for purposes of illustration.

FIG. 1A illustrates the general outline of a cargo airship 100 equipped with a payload bay door or flap 120 on its upper surface. The cargo airship 100 could also be described as a “low-altitude airship”, “first stage airship”, “launch airship”, “logistics tug”, or other appellations. A payload bay, which is located inside the airship 100, contains a high-altitude airship intended to be launched using the inventive concept. The payload bay communicates with the outside environment via the payload bay door or flap 120. The cargo airship 100 includes sufficient buoyant volume, separated from the payload bay, to carry itself and its payload to a desired deployment altitude. The cargo airship 100 may be manned, unmanned, or optionally manned. The cargo airship 100 may include other subsystems such as energy or fuel storage subsystems, propulsion subsystems, aerodynamic control subsystems (such as lifting surfaces and/or aerodynamic control surfaces), buoyancy control subsystems including releasable ballast, and communications, navigation, and control subsystems appropriate to the manning concept employed. The payload includes the high-altitude airship intended to be launched, support hardware, and optionally, a source of lifting gas for the high-altitude airship such as e.g. tanks of compressed hydrogen or helium, or a reservoir of chemical stocks that can be used to generate hydrogen gas at will.

The terms “low-altitude” and “high-altitude”, and their variants, are intended to indicate the relative intended (i.e., “design”) operating altitudes of the two airships including elements of the inventive system. In many examples, the “low-altitude” airship is intended to operate in the troposphere or lower stratosphere, and the “high-altitude” airship is intended to operate at a substantially higher altitude than the “low-altitude” airship. In one example, the low-altitude airship is intended to operate in the lower stratosphere at an altitude of roughly 20 km, and the high-altitude airship is intended to operate within an altitude range of 25-40 km.

FIG. 1B illustrates the general outline of a cargo airship 110, similar in many respects to the cargo airship 100 previously illustrated, equipped with a payload bay door or flap 130 on its upper and lateral surface.

FIG. 1C is a side view of a cargo airship 100 that has been partially cut away to show storage of a high-altitude airship 200 and launching apparatus (122, 124, 126) in a payload bay. The cargo airship 100 includes a number of internal ballonets 128. The ballonets 128 provide containment of the lifting gas and allow for the distribution of lifting gas to be shifted to achieve more desirable flight characteristics. In this example, a space between the ballonets 128 may be used to store: the high-altitude airship 200 on a reel 126; supplies of lifting gas 124; as well other gear such as command and control equipment 122. The payload bay door 120 is opened to allow the high-altitude airship 200 to be deployed. As discussed below in greater detail, the hull or ballonets of the high-altitude airship can be inflated from the supplies of lifting gas 124 and/or from the lifting gas in the ballonets 128 of the cargo airship 100. The reel 126 rotates to allow the high-altitude airship 200 to be deployed upward through the payload bay door 120 of the cargo airship 100 as shown in FIG. 1D. FIG. 1E is a side view of a high-altitude airship 200 at altitude with an optional payload 204 suspended from the airship by shroud lines 202. In this implementation, the high-altitude airship 200 includes a “pumpkin lobed” hull balloon intended for operation in the upper stratosphere.

FIGS. 2A and 2B illustrate the general outline of one type of high-altitude airship 200 that can be deployed using the inventive concepts disclosed herein. As shown in FIGS. 2A and 2B, an inflation tube 210 runs from a point near the crown of the airship to a point near the base or collar of the airship. Only a portion of the tube is shown in FIG. 2A.

In FIG. 2B the airship 200 has been illustrated as transparent to show the full length of the tube 210. The inflation tube 210 allows lifting gas to be selectively introduced into the crown of the airship during initial inflation of the airship. By inflating the crown of the airship first, the airship deployment proceeds smoothly and allows the reel to continuously unroll the high-altitude airship as the crown of the airship rises out of the payload bay 120, FIG. 1D. If the high-altitude airship 200 is constructed along the lines of a pumpkin-lobed balloon as shown in FIGS. 2A and 2B, the inflation tube 210 may be integrated with one of the seams between gores of the balloon. However, other integration methods could be used, including supporting the inflation tube 210 from the crown of the airship so that the tube hangs down the center of the airship when the airship 200 is inflated. The purpose of the inflation tube 210 is to allow hydrogen gas to be introduced in such a way that the crown of the airship is the first portion to be significantly inflated, as opposed to its base (more generally, the purpose of the inflation tube is to first inflate the part of the high-altitude airship or balloon that is outermost on the drum or spindle 250, and which will be deployed first).

FIG. 3A illustrates a general stowage concept for a high-altitude airship carried within the payload bay of the cargo airship 100 (or 110), in accordance with one example of the present specification. This particular example is adapted to the form of the airship 200 illustrated in FIG. 1A-1D, but may be used for certain other airship types and geometries as well. As illustrated in FIG. 3A, the hull including the envelope of the lifting volume of the high-altitude balloon or airship 200 is wound loosely on a reel 255. In this example, the reel 255 includes a spindle 250 between end plates 260 and 270. The reel 255 may be supported by additional equipment that is not shown and may include a variety of additional components including motors, brakes, and sensors.

The inflation tube 210 is constructed to resist crushing so that it maintains an open cross-section along its entire length when subjected to the expected compression loads associated with the hull of the high-altitude airship 200 (along with the inflation tube 210 itself) being wound on the reel 255. As the airship 200 is wound onto the reel 255, tension in the direction of winding is carefully managed to insure that compressive forces do not crush the inflation tube 210.

The inflation tube 210 is connected, through the action of a commandable valve that is part of the balloon 200, and a reversible mating apparatus, to a filling port on the reel 255. The filling port, in turn, is fluidically connected to an external supply of lifting gas. One or more of these connections includes a rotating joint (such as a slip ring joint) that allows a first half of the connector to rotate with the reel 255. The other portion of the connector is connected to the external gas supply and remains stationary.

The inflation tube 210 may include one or more diffusers on its terminal end to ensure that hydrogen gas is distributed into the crown of the balloon without damage to the balloon material by the temperature or pressure of the hydrogen. For example, if the hydrogen is taken from cryogenic storage, the hydrogen may be very cold. The diffuser allows the hydrogen to be more effectively distributed and warmed. The diffuser may include an end cap with multiple openings, multiple openings along the length of the tube, or other appropriate configuration. The diffuser may allow the hydrogen to be delivered at higher pressures for more rapid inflation of the balloon. Further, if the tube is used to extract gas from the balloon, the diffuser provides multiple openings that are less likely to be blocked.

In operation, the apparatus and structures illustrated generally in FIG. 3A allows the high-altitude airship 200 to be filled with lifting gas even while it is being unspooled from the drum or spindle 250. Specifically, after the cargo airship 100 (or 110) reaches a desired deployment and launch altitude, and is allowed to drift with the wind so that it achieves close to zero airspeed, the payload bay door or flap 120 is opened or retracted, exposing the payload bay with its high-altitude airship 200 wound on the reel 255. A small amount of lifting gas is introduced into the high-altitude airship 200 by way of the previously-noted plumbing contained in the drum or spindle 250, the filling port, the reversible mating apparatus, the commandable valve, and the inflation tube 210. This causes the crown of the high-altitude airship (or other portion that is wound outermost on the drum or spindle 250) to inflate, become buoyant, and start to pull upward and away from the reel 255. This will naturally tend to cause the drum or spindle 250 to unwind. Additional lifting gas can be introduced as already described, taking care to avoid an excessive inflation rate that could cause the hull of the high-altitude airship to rupture. A motor and brake apparatus, associated with the reel 255 is included in some examples to enhance the deployment sequence by assisting in initial deployment (through motorized turning of the drum or spindle) and also slowing the rate of deployment (braking, if necessary).

As illustrated in FIG. 3A, even in its “stowed” or “wound” configuration, a small portion of the high-altitude airship may remain substantially “unwound” on the drum or spindle 250, so as to facilitate initial inflation and the creation of a suitable “bubble of gas” that will achieve positive buoyancy (so that it floats upward and away from the drum or spindle 250).

In another example, an inflation tube or functionally similar structure is connected to a portion of the high-altitude airship that is accessible or “exposed” on the outer layer of the high-altitude airship as it is wound on the drum or spindle 250. In another example, the inflation tube is connected to a portion of the high-altitude airship that is initially not wound on the drum or spindle, and this portion of the high-altitude airship is also releasably secured to a fixture or stay on the cargo airship. For example, an equipment package of the high-altitude airship including a “fill port” with a commandable valve is releasably secured on the outer surface of the cargo airship, while the majority of the hull of the upper airship is spooled on a drum or spindle inside a cargo bay of the cargo airship or alternatively folded in a configuration to facilitate proper deployment. Upon initial inflation with lifting gas via the fill port, a “bubble” of lifting gas will form inside a portion of the hull of the high-altitude airship that is in close proximity to the fill port, and will serve to extract the remainder of the high-altitude airship from the cargo bay. Following proper inflation, the commandable valve is commanded to a closed position, the connection to the inflation tube is severed, and the high-altitude airship is released.

In yet another example, a long tube is connected to a fill port with a commandable valve located on an externally-accessible portion of the high-altitude airship, and this portion is allowed to lift away from the cargo airship during the inflation process. This example uses a longer filling tube while avoiding the use of a separate releasable structure securing the equipment package and fill port to the cargo airship.

As the high-altitude airship 200 is unwound from the drum or spindle 250, and inflated with lifting gas, the combined system of the cargo airship 100 (or 110) and high-altitude airship 200 becomes more buoyant (i.e., since the total mass is the same but the total lifting volume filled with lifting gas is increased), and so it tends to seek a higher altitude. The local wind field may change for this and other reasons, but the combined system tends to drift with the lateral wind (although there is a lag in responding to changes in the wind field) and airspeed tends to remain low. This has the effect of protecting the high-altitude airship 200 from damage due to high winds during deployment. When applicable, the cargo airship 100 (or 110) can be operated (maneuvered) to actively follow the winds, using its maneuvering and propulsion capability to overcome its own inertia and thereby help to minimize the local airspeed experienced by the high-altitude airship 200 during its deployment. Furthermore, in some examples, the cargo airship 100 (or 110) can actively control its own buoyancy, for example by dropping ballast, venting lifting gas or pumping lifting gas into a reservoir (thereby allowing its lifting volume to be reduced). Alternatively, propulsive forces (such as propellers or fans adapted to provide a downward force) can be utilized to counteract the increase in buoyancy. In such examples, the altitude of the combined system can be held relatively constant during the deployment sequence.

The amount of lifting gas introduced into the high-altitude airship can be metered to ensure that a proper amount of lifting gas is introduced to take the airship to its desired altitude, while guarding against over inflation. When the proper amount of lifting gas has been introduced, and the high-altitude airship is fully unspooled from the drum or spindle 250, the commandable valve is closed and the reversible mating apparatus is operated to demate the airship 200 from the mating fixture of the drum or spindle 250. The high-altitude airship then ascends from the cargo airship 100 (or 110), while the cargo airship 100 (or 110) itself descends to a lower altitude.

If the high-altitude airship 200 is associated with a payload, perhaps connected to the airship with shrouds so that the payload will be suspended below the airship during operation, the apparatus of FIG. 3A can be modified or adapted as illustrated in FIGS. 3B and 3C. Here, end plate 270 has been removed from the illustration for clarity (although it is present in the intended apparatus) and drum or spindle 250 has been replaced with fixed cylindrical segments 252 and 257 and movable cylindrical segments 254 and 259. The movable segments 254 and 259 are connected to the fixed segments 252 and 257 through hinge lines 253 and 258 and can open outward under the influence of remotely commandable latches, springs, or other actuators. The payload 204 is secured to the fixed cylindrical segments 252 and 257, and held within the generally cylindrical space as illustrated, by remotely commandable, reversibly latchable mounting apparatus not explicitly shown. Shroud lines 202 are illustrated schematically as extending from the payload 204 and wrapping around the fixed and movable cylindrical segments already described. Numerous turns of the shroud lines 202 on the cylindrical segments could exist, followed by the high-altitude airship itself. As noted previously, the high-altitude airship 200 includes an inflation tube 210, commandable valve, and reversible mating apparatus which in this example is connected to a filling port 280 located axially on the end plate 260. A variety of additional components may also be included. For example, a short tube may be used to span the distance from the airship hull itself, which may be outside the generally cylindrical space bounded by the cylindrical segments 252, 254, 257 and 259, and the reversible mating apparatus connected to the filling port 280.

In operation, the high-altitude airship 200 is unwound from the apparatus illustrated in FIGS. 3A-C. However, the unwinding activity is paused when the high-altitude airship 200 is clear of the apparatus. The commandable valve is then closed. In one example, a hull integrity check can be performed at this stage and, if a breach in the hull is detected, the high-altitude airship can be vented of its lifting gas and rewound on the illustrated apparatus, and returned to the ground for repair (alternatively, if it is infeasible to re-stow the airship, it can be jettisoned and only the payload can be returned to the ground). If no fault is detected, the reversible mating apparatus is operated to demate from the filling port 280. The unwinding activity is then continued, until the shroud lines are fully extended. At this stage, the rotation of the apparatus is again halted. The movable segments 254 and 259 are commanded open and pivot on hinge lines 253 and 258, respectively, as shown in FIG. 30. The remotely commandable latches holding the payload 204 in place are then commanded open, releasing the payload 204 and the entire high-altitude airship 200 from the cargo airship 100 (or 110).

Other Examples

In another example, instead of a rotatable reel, the high-altitude airship is folded and stowed in an accordion fashion within the payload bay of the cargo airship. FIG. 4 illustrates a partially-filled high-altitude airship at an early point in the deployment process associated with such an example. The high-altitude airship 200 is shown partially folded and partially inflated. The figure illustrates a reservoir of lifting gas 420, a remotely operable valve 422, and a gas line 425 fluidically connecting the valve 422 to a remotely operable mating apparatus 430 which is mated to a gas fill port on the collar 440 of the high-altitude airship 200. The collar 440, in turn, includes a second remotely operable valve (not shown) connecting the gas fill port to the inflation tube 210, which was previously discussed. Also shown is a bracket 450 and remotely operable latch 475 which holds the collar of the high-altitude airship in place until the latch 475 is released. The payload 204 is attached to the collar 440 by shroud lines 202. In operation, the remotely operable valves are operated to fill the airship with a suitable amount of lifting gas from the reservoir 420. The high-altitude airship inflates and extracts (or substantially extracts) itself from the cargo bay. The remotely operable valves are closed and, in one example, a hull integrity check is performed. The remotely operable mating apparatus 430 is operated to demate from the collar of the high-altitude airship, and the remotely operable latch 475 is operated to release the high-altitude airship 200 along with its payload 204.

In another example, the high-altitude airship is secured to the top of the cargo airship rather than being stowed internally within a payload bay. In this example, if the high-altitude airship is associated with a payload, the payload may be stored in a payload bay, or it also may be secured externally to the cargo airship and released at a later time.

Deployment of a Tethered Airship System

In the implementation described below, the numerical examples are approximate and intended to illustrate general principles of the inventive concept as opposed to a precise design. Further, a number of simplifying assumptions, such as ignoring temperature differences, are used in the illustrative calculations.

Consider a tethered airship system, similar to the system described in U.S. application Ser. No. 13/159,215 (now U.S. Pat. No. 8,864,063), including at least a lower airship, an upper airship and a tether connecting the two airships. In an example of the present description, the lower airship includes or supports a primary payload intended to be returned to Earth through the action of a releasable payload return vehicle such as a winged glider or powered aircraft. The lower airship may also include or support a parachute for drag enhancement, a controllable reel assembly to control the length of deployed tether extended to the upper airship, and/or other aerodynamic control surfaces or structures including lifting surfaces, elevators, rudders, airbrakes, and the like. The upper airship supports the tether and furthermore includes or supports structures or mechanisms to adjust lift and drag, such as e.g. one or more of a parachute, parafoil, and mechanisms to control airship angle-of-attack. In one example described in the present specification, the upper airship and tether is initially launched from the ground and carried as a secondary payload of the lower airship. As one numerical example of the principles described herein, assume that the lower airship (with its primary payload and payload return vehicle) weighs ˜5000 kg, is designed to operate in an altitude range of 18 to 19 km, and includes a maximum lifting volume of 44,700 m³ in one or several ballonets that can withstand a maximum pressure difference of 2 kPa (i.e., they can be deflated, or partially inflated, or fully inflated to ambient pressure, or fully inflated to a pressure up to 2 kPa above ambient pressure). Assume the upper airship weighs ˜2150 kg, is designed to operate in an altitude range of 27 to 37 km and includes a maximum lifting volume of 500,000 m³ in one or several ballonets that can be inflated to a maximum pressure difference of 100 Pa relative to ambient pressure. In an example, the upper airship can maintain a desired aerodynamic shape by transferring lifting gas between internal ballonets and filling non-lifting internal volumes with ambient air under at least some operating conditions such as e.g. when the upper airship is below its maximum operating altitude and experiences non-zero airspeed. Assume that the tether weighs 600 kg.

Initially, the total weight of all systems is 7750 kg and the lower airship, considering its maximum lifting volume, can carry this total weight to an altitude in excess of 15 km. At this altitude, atmospheric density is about 0.193 kg/m³ and the density of an equivalent volume of hydrogen is about 0.0015 kg/m³. If this is rounded up to 0.002 kg/m³, the reader will appreciate that the lifting capacity of the lower airship will be about 0.191 kg/m³ and only 44,000 m³ of lifting volume would be sufficient to carry 8400 kg (650 kg more than the total system weight) to 15 km. Hence the lower airship can carry the total system to 15 km with some design margin (i.e., the ballonets may not be fully inflated at 15 km altitude in order to carry the total system weight).

With the system as described, the total system is in buoyant equilibrium at about 15 km altitude with most or all of the lifting volume of the lower airship operated at ambient pressure (i.e., some or all of the ballonets may be partially inflated, and pressurized sections of the hull intended to maintain a rigid aerodynamic shape have been ignored for this altitude calculation). In an example, alternatively, the lifting ballonets can be arranged near the outer surface of the airship, and structured such that they can be pressurized to maintain a desired aerodynamic shape of the lower airship. In this alternative example, non-lifting volumes of the airship can be filled with air at substantially ambient pressure. In yet another example, a desired aerodynamic shape of the lower airship is maintained by a combination of pressurized lifting ballonets and pressurized non-lifting (air-filled) ballonets, and additional non-lifting volumes of the airship are filled with air at substantially ambient pressure. In any case, the lifting gas may be transferred from one lifting ballonet to another within the lower airship, at constant pressure, without affecting total system buoyancy. Similarly, some of the lifting gas could be transferred at atmospheric pressure from the ballonets of the lower airship, to the upper airship, without significantly affecting total system buoyancy (note: if the lifting ballonets of the lower airship are pressurized, then transfer to the upper airship at atmospheric pressure will result in a slight increase in total system buoyancy). Hence, the upper airship could be filled with a sufficient quantity of lifting gas to carry the upper airship and the full weight of the tether to the maximum design altitude of 37 km, where the lifting volumes of the upper airship would be fully inflated and would experience an internal pressure roughly 100 Pa above ambient. However, prior to release, the upper airship and lower airship are both at or near the pre-existing equilibrium altitude since the total lifting volume has not significantly changed (it has only been shifted from one part of the system to another). The volume of gas, at ambient pressure at 15 km used to carry the upper airship plus tether to that altitude is roughly (2150+600)/0.191≈14,400m ³, or roughly one-third of the original lifting volume of the lower airship. If carried to an altitude of 37 km, and allowed to expand to match atmospheric pressure at that altitude (420 Pa), this volume of lifting gas would expand to roughly 415,000 m³. If instead, a volume of approximately 21,600 m³ of lifting gas is transferred (at 15 km where pressure is nominally 12,045 Pa), the upper airship could be operated at 37 km at its full lifting volume of 500,000 m³ and full internal pressure of 520 Pa (i.e., 100 Pa above ambient).

After a transfer of 21,600 m³, the lower airship has an effective lifting volume of roughly 22,400 m³ or less, considering that it started with less than 44,000 m³. Hence, its lifting capacity is no more than (22,400 m³)*(0.191 kg/m³)≈4300 kg. This is less than the weight of the lower airship (5000 kg). If the upper airship is released and allowed to ascend to 37 km altitude, it will provide a buoyancy force of (500,000 m³)*(0.0055 kg/m³)=2750 kg; roughly the weight of the upper airship plus the tether. So the total lifting force is no greater than 6950 kg, which is less than the total system weight (7800 kg). Hence, the lower airship will not be able to maintain its altitude as the upper airship ascends—additional lifting gas may be added to the lower airship in order to increase buoyancy, and actually achieve the intended operational altitude of about 18 to 19 km. In order for the lower airship to carry its own weight ignoring the tether, at an altitude of 18 km where atmospheric density is 0.1207 kg/m³ and atmospheric pressure is 7,505 Pa, and the lifting capacity of hydrogen is roughly (13/14)*0.1207˜0.112 kg/m³, the lower airship may attain a lifting volume of roughly 44,600 m³. This same amount of lifting gas, at the initial deployment altitude of 15 km where atmospheric pressure=12045 Pa, includes a volume of roughly (44,600 m³)*7505/12045˜27,800 m³.

Hence, considering the remaining complement of the lower airship (roughly 22,400 m³ of lifting gas after the inflation of the upper airship), roughly 5400 m³ of hydrogen gas may be added at the ambient atmospheric pressure of 12045 Pa. This can be added from an internal reservoir carried by the lower airship (cryogenic, non-cryogenic, or chemical), or an external tanker/logistics airship adapted to provide additional lifting gas.

In the example just described, some of the use of additional lifting gas could be alleviated by utilizing the full lifting volume of the lower airship. It may also be feasible, in some examples, to tailor the initial operation of the lower airship so that it experiences a positive pressure differential (inside pressure compared to outside) at equilibrium altitude prior to the transfer of lifting gas to the upper airship. This would leave the lower airship with a slightly greater volume of lifting gas following the transfer of gas to the upper airship (referenced to ambient atmospheric pressure), thereby minimizing the use of replenishment gas from an internal reservoir or external tanker/logistics airship. However, care should be taken when performing such an operation since the tendency, upon initiating the transfer of gas, will be for the total system to rise slightly in altitude (i.e., since the lifting volume of the upper airship increases, but the lifting volume of the lower airship remains unchanged since its lifting volume is initially pressurized relative to ambient pressure). Thus, care should be taken to ensure that the design parameters of the lower airship (specifically, maximum pressure differential) are not exceeded, in any of its ballonets or its lifting volume as a whole, during inflation of the upper airship.

In one example, the lower airship is augmented with a cryogenic reservoir of lifting gas (which increases its weight), loaded with the upper airship and its associated deployment apparatus, and inflated to achieve neutral buoyancy at full ballonet inflation, but without overpressure, at an altitude below its intended operating altitude. Following the transfer of lifting gas to the upper airship, and deployment of the upper airship to a higher altitude, the lower airship replenishes its lifting volume with lifting gas from the cryogenic reservoir. The cryogenic reservoir may be retained on the lower airship for the life of the mission, or it may be jettisoned in a safe area where it will not pose a risk to life or property on the ground (e.g., an oceanic area). In an example where the cryogenic reservoir is jettisoned, the weight of the empty reservoir represents ballast which would tend to reduce the equilibrium altitude of the lower airship prior to being jettisoned (and specifically, during the transfer of lifting gas from the lower airship to the upper airship). At the lower equilibrium altitude, the ballonets at full inflation hold a larger quantity of lifting gas (i.e., a larger mass of lifting gas) than they would at a higher equilibrium altitude. Thus, after transferring a desired quantity of lifting gas to the upper airship, a greater amount of lifting gas remains in the lower airship, thereby reducing the size of the cryogenic reservoir used to “top off” the lifting volume of the lower airship.

If a reservoir of lifting gas is available during gas transfer to, and inflation of, the upper airship, or equivalently if a tanker/logistics airship is available during this process, there could also be a transfer directly from the reservoir (or tanker/logistics airship) to the upper airship, for at least some of the lifting gas, thereby avoiding the depletion and, subsequently, replenishment of the lower airship.

In another example, the transfer of lifting gas from the lower (cargo) airship to the upper airship is performed at an altitude where the total amount of lifting gas, used by both airships at their ultimate operational altitudes, can be accommodated within the lifting volume of the lower airship. For example, consider a tethered airship system as before where the lower airship weighs ˜5000 kg and is designed for operation at 18 to 19 km with a total lifting volume of 44,700 m³, the upper airship weighs ˜2150 kg and is designed for operation up to 37 km with a total lifting volume of 500,000 m³, and the tether weighs 600 kg. If the transfer is performed at an altitude of 10 km, ambient atmospheric pressure is approximately 26.5 kPa and the internal pressure of the lower airship can be maintained at 27.5 kPa using internal ballonets filled with atmospheric gases to an overpressure (relative to ambient) of 1 kPa. Under these conditions, all the lifting gas for both airships is compressed into a volume of approximately 25,000 m³. This is substantially less than the total lifting volume of the lower airship which, as noted above, is 44,700 m³. So no extra storage facilities, cryogenic or otherwise, are used. However, the lifting capacity of hydrogen gas at this altitude, held at 1 kPa overpressure, is roughly 383 g/m³. Consequently, the total lifting capacity is roughly (25,000 m³)·(0.383 kg/m³)≈9550 kg which exceeds the total system weight by roughly 1700 kg. As a result, the system is positively buoyant at 10 km altitude and will tend to rise. In this example, the transfer may be completed quickly in order to avoid risk of rupturing the lifting ballonets of the lower airship. This risk can be mitigated by adding an additional releasable mass or ballast to the lower airship, where it is understood that the releasable mass has a density greater than air. For example, if 1850 kg of water ballast is initially carried aloft, the combined system will be neutrally buoyant at less than 10 km altitude prior to the transfer and will be neutrally buoyant at roughly 10 km altitude after the transfer (i.e., considering the lower density of lifting gas in the upper airship versus the lower airship), and the transfer of lifting gas can be performed in a leisurely manner without undue risk. The water ballast can be released after the transfer is complete, in order to allow the tethered airship system to achieve its intended operational altitudes (i.e., in this example, 18 to 19 km for the lower airship, and up to 37 km for the upper airship). If excess water ballast is initially carried aloft, it can be “metered out” in a controllable release in order to achieve a desired equilibrium altitude for the transfer.

In one example, in lieu of some or all of the water ballast, the lower airship further includes a detachable (releasable) unmanned air vehicle, such as an autonomous or remotely-controlled glider, containing a pump which is adapted and plumbed to facilitate the transfer of lifting gases from the lower airship to the upper airship. The unmanned aerial vehicle represents a releasable mass which is carried aloft by the lower airship. Following the transfer, the connections to the lower airship are severed (with commandable valves having been commanded closed to avoid the unintended release of lifting gas), the unmanned air vehicle is released, and it is flown back to a selected landing or recovery site for a safe recovery (alternatively, it may be “ditched” in a safe area such as a mid-ocean area devoid of ships). This example is particularly desirable when a pump is used to facilitate the transfer of lifting gas, but where the pump is not used subsequent to the transfer and would therefore represent “dead weight” carried by the lower airship during its operational mission.

In order to avoid the use of ballast tanks and valves on the lower airship, which would also represent “dead weight” during the operational mission, the unmanned air vehicle can carry all the extra ballast to be used by the system. The unmanned air vehicle can drop the ballast before it is itself released from the lower airship, or after said release.

The unmanned air vehicle can perform other desirable functions. For example, if the unmanned aerial vehicle is exposed to the airstream while it is attached to the lower airship (prior to its release), it may provide at least some aerodynamic control. Specifically, the orientation of its wing or wings can be adjusted relative to the lower (cargo) airship and the ambient wind field in order to vary the horizontal or vertical forces induced by the wind. These varying forces can be used, in turn, to vary the angle of attack or angle of side slip of the lower (cargo) airship, or to provide lateral steering forces. In an example where the unmanned aerial vehicle is powered (e.g., includes a propeller), it can also provide some propulsion to the system prior to its release from the lower airship.

In another example, the low-altitude/cargo airship includes a cargo bay that extends vertically through the entire airship, with doors, hatches, or openings on the upper surface and lower surface of the hull. The upper airship's deployment mechanisms, a pump to facilitate the transfer of lifting gases, and additional ballast, are assembled into a detachable subsystem that can be released and jettisoned through the lower door, hatch or opening. The detachable subsystem includes a releasable mass. Following the successful deployment of the upper airship, and after closing valves to prevent the undesired release of lifting gas, the detachable subsystem is released and jettisoned. It can be allowed to fall into the ocean or an uninhabited area with or without a parachute, or it can be adapted to deploy wings and fly back to a desired recovery site.

The upper airship of a tethered airship system may include multiple elements such as a buoyant element (such as a high-altitude balloon) at the high end of a tether, a parafoil, a parachute, and a docking element for a shuttle designed to move up and down the tether, with the parafoil, parachute, and docking element attached (and controlled from, as applicable) subsystems spaced apart along the tether. The tethered airship system may also include elements such as the aforementioned shuttle that moves up and down the tether, and elements associated with the lower airship such as tether attachment/deployment mechanisms and sheaths designed to protect other elements of the system from abrasion and wear. In some examples, all of these elements may be “stacked” in accordion fashion prior to deployment, in a generalization of the stowage concept illustrated in FIG. 4. In other examples, the upper airship, its associated parafoil, parachute, docking elements (and their attachment and control apparatus) and a portion of the tether are stowed on a rotatable spool as indicated generally in FIG. 3. As part of the deployment sequence, the lower end of the tether deployed from said rotatable spool is transferred and attached to the end of another segment of tether which is associated with the rest of the system. In still other examples, the rotatable spool used for stowing and deploying the upper airship and its associated elements is integrated with the “high-altitude” docking element for the shuttle (this docking element being the lower-most mechanical element associated with the upper airship). The docking element is in turn attached to the end of another segment of tether which is associated with the rest of the system. All attachments are made on the ground and verified prior to launch. This example avoids the engineering challenge and mission risk of making and ensuring a secure attachment between two elements during the deployment sequence.

The specific details described above are merely examples. The principles described herein are not limited to the examples disclosed. The principles could be used with a wide range of airships and in a wide variety of configurations. In general, a method for launch of an airship includes connecting a cargo airship to a deflated (or partially inflated) second airship that is not positively buoyant at the launch site, and launching the cargo airship. The cargo airship carries the second airship aloft and inflates the second airship with lifting gas carried by the cargo airship. The second airship is then released from the cargo airship. FIGS. 5 and 6 are flowcharts that describe methods for launching airships from a cargo airship.

FIG. 5 is a flow chart describing a method 500 for deploying a high-altitude airship from a cargo airship. The upper airship is stowed in the cargo airship (block 505). The cargo airship is launched and travels to a designated location (block 510). Deployment of the upper airship begins by opening the payload bay and opening a valve to allow lifting gas to flow through an inflation tube into the crown of the upper airship (block 515). The crown of the upper airship inflates and progressively lifts out of the payload bay as more lifting gas passes through the inflation tube (block 520). The upper airship receives the desired amount of lifting gas, lifts its payload from the payload bay of the cargo airship, and ascends to the desired altitude (block 525).

The method described above is merely one example and could be altered in a variety of ways. The blocks could be deleted, added, combined or reordered. For example, additional blocks could be added to describe shutting the valve after the desired amount of lifting gas has been received by the high-altitude airship or opening a reel to release the payload from the payload bay.

FIGS. 6A and 6B show a more detailed method 600 for deploying a high-altitude airship from a cargo airship. In this example, the high-altitude airship and its payload (if any) are stowed in a payload bay of the cargo airship (block 605). As discussed above, the hull of the high-altitude airship may be stowed in a variety of configurations, including wrapped around a reel or a number of folded configurations. The payload and other items, such as parachutes, tethers, parafoils, and other items, can be appropriately attached to the hull and stowed for deployment out of the payload door in an upper surface of the cargo ship. The high-altitude airship includes a tube that passes from a source of lifting gas through the length of the uninflated hull to the crown of the hull.

The cargo airship is launched and travels to a predetermined altitude and location (block 610). In some examples, the cargo airship may use a combination of buoyant lift, vectored thrust, ballast, aerodynamic lift, and pressure control of ballonets to maintain the desired heading, altitude, and velocity. For example, if the total mass of the cargo airship and its payload exceeds the buoyant lifting capacity of the cargo airship, the cargo airship could use vectored thrust and aerodynamic lift to increase its lifting capacity. The vectored thrust could be generated in a variety of ways including the use of directional ducted fans. Aerodynamic lift can be created through the geometry and/or angle of attack of the airship and lifting surfaces attached to the airship or an associated aerial vehicle, such as an unmanned aerial vehicle intended to be dropped or jettisoned from the cargo airship at a later time. The attitude of the cargo airship can be adjusted through the use of aerodynamic control surfaces on the airship or an associated aerial vehicle, vectored thrust, and/or shifting of lifting gas between ballonets.

The cargo ship then transitions into drifting flight while launching the high-altitude airship (block 615). Drifting flight can include a variety of maneuvers and propulsion schemes that minimize the speed of winds on the upper airship below a predetermined threshold as it is being inflated and launched. For example, drifting flight may include controlling the cargo ship so that it travels approximately at the same speed and in the same direction as the surrounding air. Drifting flight may also include using propulsion to alter the speed or orientation of the cargo airship to maintain the desired heading. In some circumstances, drifting flight may include slight differences between the speed of the cargo airship and the surrounding air. These slight differences may be used to apply desirable aerodynamic forces on the high-altitude airship intended to facilitate deployment.

The launch of the high-altitude airship includes opening a payload door in the upper surface of the cargo airship and opening a commandable valve to allow lifting gas to flow through the inflation tube into the crown of the high-altitude airship hull (block 620). It may also involve the use of a pump to facilitate the transfer of lifting gas. As the hull inflates the high-altitude airship is progressively deployed out of the payload bay (block 625). When the desired amount of lifting gas has been transferred from a lifting gas reservoir to the upper airship, the commandable valve is closed (block 630).

Continuing in FIG. 6B, a hull integrity check is performed (block 635). The hull integrity check may include a variety of sensors and techniques designed to determine if a hole or a rip in the hull is allowing lifting gas to escape. The hull integrity check may include sensing pressure changes in the hull, imaging the interior or exterior of the hull, detecting a change in the lifting capacity of the high-altitude airship, or other appropriate technique.

An evaluation of the hull integrity is then made (block 640). If the evaluation indicates that the hull integrity is not satisfactory, the high-altitude airship can be jettisoned or the lifting gas can be removed and the high-altitude airship restowed in the payload bay (block 645). Jettisoning the high-altitude airship may include actions such as firing a pyrotechnic cutter to cut the shroud lines that attach the hull of the airship to the payload. The hull and the lifting gas remaining inside the hull then rise rapidly away from the cargo airship. Restowing the high-altitude airship allows the airship to be returned to the ground for repair or replacement. The lifting gas can be removed using a variety of techniques, including venting the lifting gas and/or pumping the lifting gas out of the hull. Lifting gas that is pumped out of the hull can be stored in pressure tanks or returned to ballonets in the cargo ship. After the damaged hull of the upper airship has been jettisoned or restowed, the cargo airship can return to the ground for replacement or repair of the high-altitude airship (block 650).

If the hull integrity is determined to be satisfactory (“Yes”) the payload can be released from the cargo airship to allow the upper airship to ascend (block 655). The high-altitude airship may be free flying or may be connected to the cargo airship by a tether. After deployment of the high-altitude airship, there may be one or more lifting gas reservoirs that are empty. These depleted lifting gas reservoirs can be jettisoned to improve the endurance and buoyancy of the cargo airship (block 660).

In sum, a two-stage deployment concept that includes a robust cargo airship that can carry a high-altitude airship as a payload, travel to a desired deployment location and deploy the high-altitude airship at a suitable altitude. This enables direct insertion into difficult environments such as polar or mid-oceanic areas. By deploying the upper airship while the cargo airship is in drifting flight, winds can be minimized. This allows for easier launch and lower damage risk to the upper airship. The cargo airship can be launched in a wide variety of wind conditions and sites. This greatly increases the available launch windows for upper airships.

Further Discussion of Systems With Releasable Masses

In an example, the releasable mass (such as an unmanned aerial vehicle) may include a payload which may be considered to be a primary payload intended for extended operation while the releasable mass remains attached to the lower (cargo) airship. In this example, the releasable mass is not released or jettisoned immediately following deployment of the high-altitude airship, but instead, remains attached (unreleased) for an extended period including days, weeks, or even years. In an example where the high-altitude airship remains tethered to the cargo airship (and may be considered to be a “secondary payload” of the cargo airship), the tethered airship system as a whole provides station-keeping or maneuvering capability intended to support the mission for which the primary payload is adapted. Exemplary primary payloads include communications relay payloads, earth observation payloads including multispectral sensors, astronomical instrument payloads (such as infrared telescopes and sensors beneficially carried above the majority of the Earth's atmosphere), remote surveillance payloads adapted to civilian or military missions (such as radar or lidar), signals intercept payloads, and weaponry payloads such as kinetic or directed-energy weapons. In some systems, the primary payload is adapted to operate while the releasable mass remains attached to the lower (cargo) airship. In other systems, the primary payload is adapted to operate after the releasable mass is released from the lower (cargo) airship.

The primary payload in a system as described herein may represent a significant financial investment, and furthermore, such a primary payload may be relatively heavy and represent a risk to human life or property if it falls to Earth in an uncontrolled manner. The history of safely returning large stratospheric airships to the Earth, with successful landings that preserve the value of attached hardware, is unsatisfying. Therefore, by placing such a high-value (and potentially heavy) primary payload on a releasable unmanned aerial vehicle that can independently maneuver to a landing on land or at sea, the primary payload can be recovered with high probability, and the risk to human life and property is reduced.

As an alternative to a landing on land or at sea, the unmanned aerial vehicle can be adapted to be recovered by flying into a vertically-suspended line or cable, as has been demonstrated by Boeing/Insitu's ScanEagle aircraft, or a net. As another alternative to a landing on land or at sea, the unmanned aerial vehicle can be adapted to deploy a parachute for recovery on land or at sea, or even while descending through the air by parachute.

If the lifting and aerodynamic control surfaces of the unmanned aerial vehicle are exposed to the external wind field while the unmanned aerial vehicle remains attached to or integrated with the lower (cargo) airship, the unmanned aerial vehicle can be employed to assist in the maneuvering and control of the cargo airship (and the tethered airship system of which it may be a part). For example, the unmanned aerial vehicle can induce pitch and yaw forces on the lower (cargo) airship, and these pitch and yaw forces can be varied by varying the orientation of the unmanned aerial vehicle (specifically its lifting and aerodynamic control surfaces) relative to the lower (cargo) airship and the external wind field. Pitch forces can affect the angle of attack of the lower (cargo) airship, and thereby affect the lift (positive or negative) and drag experienced by the lower (cargo) airship. Yaw forces are useful for lateral steering in the local wind field and can also affect the drag experienced by the lower (cargo) airship by influencing the angle of sideslip.

If the unmanned aerial vehicle is powered, in the sense that it has a propulsion capability such as one or more propellers powered by e.g. stored electrical power (batteries) and electric motors, this propulsion capability can assist in stationkeeping during low-wind conditions occasionally experienced by a tethered airship system, maneuvering at both low and high altitudes (including provision of lateral steering forces through differential thrust—for example, if there is one propeller mounted on each of two traditional wings, and also provide for powered flight during payload return after release from the lower (cargo) airship.

In an example, a tethered airship system with attached payload (for example, as part of a detachable unmanned aerial vehicle) may draw electrical power in order to operate the payload as well as e.g. housekeeping and flight control functions of the lower (cargo) airship, telemetry gathering and data transmission, command and control functions, and flight safety functions (such as e.g. a radar transponder and ADS-B transmitter). One way to support these systems is to provide an electrical power system on the lower (cargo) airship that includes an array of solar cells and batteries for energy storage. A candidate location for the array of solar cells is the upper surface of the lower (cargo) airship. The batteries could be distributed in various ways between the lower (cargo) airship and the payload return vehicle; however, in an example, the majority (or even all) of the batteries may be placed on the unmanned aerial vehicle including the primary payload. While not generally considered to be high-value, the batteries can be heavy, and consequently, may pose a risk to humans and property on the ground in the event of an unexpected failure of the lower (cargo) airship. By placing the batteries on the unmanned aerial vehicle, they can be returned to the ground safely along with the payload; and furthermore, in the case of a powered payload return vehicle (i.e., including propulsion capability), the batteries can be used to drive the propulsion subsystem until they are nearly exhausted. In an example, some stored energy may be retained for housekeeping, navigation, telemetry, command and control, and flight control through touchdown of the unmanned aerial vehicle.

Some candidate attachment and integration methods for a detachable mass including an unmanned aerial vehicle with a primary payload are illustrated generally in FIGS. 7A and 7B. In FIG. 7A, an external view of a lower (cargo) airship 700 with a tightly-integrated payload return vehicle 750 is shown. In this illustration, most of the fuselage of the payload return vehicle is not visible outside the outer hull of the lower (cargo) airship, although the wings and tail surfaces (and a portion of the fuselage) are visible. A tether 890 ascending to an upper airship of a tethered airship system is shown coupled to the lower (cargo) airship. In the illustrated configuration, the payload return vehicle is mounted upside-down in order to minimize the risk of damaging the lower hull with the tail surfaces of the payload return vehicle. However, it is also possible to mount the payload return vehicle right-side up in an example. In FIG. 7B, a payload return vehicle is shown suspended below the lower (cargo) airship by suspension lines which may be of fixed length, or alternatively, may be controllable to allow variable length. In both integration methods, among others, the designer may be motivated by some combination of one or more of at least the following objectives: a) allowing for safe and convenient detachment/release of the payload return vehicle 750 from the lower (cargo) airship; b) allowing proper operation of the payload; c) allowing for the generation of pitch and yaw forces on the lower (cargo) airship to enable changes in lift, drag, and lateral steering forces; and d) provision of incremental positive lift or negative lift (a downward force) using the winged surfaces directly.

In a sense, the unmanned aerial vehicle (the payload return vehicle) may be considered to be a “control structure” for the lower (cargo) airship, thereby minimizing the size of traditional control surfaces used on the lower (cargo) airship itself (such as an elevator and rudder), or even eliminating them entirely. This effect is enhanced in the suspended example of FIG. 7B, where the lifting and control surfaces of the payload return vehicle are well-separated from the hull of the lower (cargo) airship and behind its overall center of gravity (CG). For example, an increase in the bow suspension line 760 of the payload return vehicle will cause a negative angle of attack on the wings of the payload return vehicle, which will pull down the rear of the lower (cargo) airship, essentially acting as an elevator for the lower (cargo) airship. This will increase the angle of attack of the lower (cargo) airship, increasing drag and generating some lift which opposes (in direction, if not strictly magnitude) the downward force generated by the wings of the payload return vehicle. Conversely, a reduction in the length of bow suspension line 760 will cause a positive angle of attack on the wings of the payload return vehicle, which will reduce the load on the rear of the lower (cargo) airship, leading to a negative angle of attack of the lower (cargo) airship, also increasing drag but generating some negative (downward) lift force opposing the upward force generated by the payload return vehicle.

If a wing suspension line 770 is lengthened or shortened, the wings of the payload return vehicle will generate lateral as well as vertical forces, thereby acting as a rudder and allowing yaw control of the lower (cargo) airship.

By leveraging the entire winged surface of the payload return vehicle in the manner described (or by using similar methods), a significant control surface is brought into play, and a structure that is used for payload return can also provide operational benefit during the primary mission while the payload return vehicle is mated (or suspended from) the lower (cargo) airship.

As an alternative to suspension lines, the payload return vehicle could be suspended below the lower (cargo) airship by rigid rods or beams that resist bending. In some examples, the effective length of such rods or beams, and/or the angle of such rods or beams to the vertical, can be adjusted so as to change the orientation of the payload return vehicle relative to the lower (cargo) airship.

In either the tight integration concept illustrated in FIG. 7A, or the suspended integration method illustrated in FIG. 7B, a powered payload return vehicle (i.e., one with propulsion capability) can be used to provide a propulsion capability to the tethered airship system of which it is a part. For example, with one electrically-driven propeller on each wingtip, such a powered payload return vehicle can provide longitudinal thrust as well as yaw forces. A single fuselage-mounted propeller (e.g., at the rear of the payload return vehicle as illustrated in FIG. 7B) can provide longitudinal thrust.

If the payload return vehicle is mounted e.g. behind the CG of the lower (cargo) airship, and the engines can be pivoted in the vertical plane, the propellers can be used to generate positive and negative pitch forces.

In yet another example, the lower (cargo) airship carries aloft two different releasable masses, such as two different unmanned aerial vehicles, one including hardware intended to facilitate launch of a high-altitude airship (either intended to be free-flying, or intended to remain tethered to the lower (cargo) airship), and a second releasable mass (such as an unmanned aerial vehicle) including a payload intended for extended operation while the second releasable mass remains attached to the lower (cargo) airship. In this example, the first releasable mass is released following deployment of the high-altitude airship. The second releasable mass (in an example including an unmanned aerial vehicle such as a glider or a powered aircraft) remains attached for an extended period. During this extended period, the second releasable mass with its payload is employed to perform a desired mission. Furthermore, as described above in relation to an example with a single releasable mass, this second releasable mass (if it includes an unmanned aerial vehicle with exposed aerodynamic control surfaces and propulsion capability) can be used to effect the flight dynamics of the lower (cargo) airship. At the end of the aforementioned extended period, the second releasable mass can be released from the lower (cargo) airship and flown back to Earth in a controlled manner. In an example, the second releasable mass may be flown back to a selected landing site on Earth.

In many missions that can be imagined (including some noted above, such as communication relay, Earth resources remote sensing, and civil or military surveillance), the payload is “Earth-directed” and placement of the payload return vehicle below the lower (cargo) airship (either suspended below, or mounted conformal with the lower surface of the lower airship) is convenient since this also places the Earth-directed payload below the airship. For missions where a payload is “sky-directed”, such as astronomy or remote surveillance of man-made space-based objects, several accommodations can be considered. In a first type of accommodation, the payload is designed to observe the sky “around” the lower airship which acts as a zenith occlusion. For this type of accommodation, a payload return vehicle (and payload) that is suspended below the lower (cargo) airship may provide relatively better observations since it would tend to minimize the portion of the sky occluded by the lower (cargo) airship, as compared to a conformal mounting technique. In a second type of accommodation, the lower (cargo) airship may support additional structures that serve to mitigate or render moot the occlusion. For example, a telescopic payload might work with a lower (cargo) airship that includes a central cavity and a rotatable mirror assembly close to the upper surface of the hull, allowing the telescopic payload to “work through” the lower (cargo) airship and observe essentially the entire sky. For this type of accommodation, a conformal mounting may allow for relatively better observations since this would cater to a more rigid mounting and a more well-defined geometric relationship between the mirror assembly and the payload in the payload return vehicle.

End of Mission

The extended period of a desired mission (using e.g. the primary payload contained within a releasable unmanned aerial vehicle) can end nominally or as the result of an e.g. catastrophic system failure.

A nominal end of mission would typically involve a planned termination wherein the lower (cargo) airship (and perhaps an entire tethered airship system) is maneuvered to a desired geographic location intended to facilitate the safe and reliable recovery of the releasable unmanned aerial vehicle along with its payload and other hardware (such as batteries, electronics, pumps, and other systems). Once maneuvered to such a location, several methods can be employed to terminate the mission. In one method, the releasable unmanned aerial vehicle is released and guided (by external control or autonomously) to a safe recovery at a selected landing site. After release of the unmanned aerial vehicle, the lower (cargo) airship, and also an upper airship if the system was configured as a tethered airship system, is/are scuttled by venting the lifting ballonets to the atmosphere. This can be performed through a vent or valve engineered into the system, or through explosive destruction of the hull(s) and lifting ballonets of the airship(s). Alternatively, in the case of a tethered airship system, the upper airship can be “reeled down” toward the lower (cargo) airship, thereby spooling the tether (or the majority of the tether) on the lower (cargo) airship. Once this is done, the tether can be severed or released/detached at either the lower or upper airship, and the upper airship (possibly with a small amount of tether still attached) can be scuttled. The lower (cargo) airship can be scuttled as well; or, if it retains aerodynamic control capability following the release of the payload return vehicle and separation from the upper airship, it could be guided to a landing or controlled crash in a desired location while venting its lifting gas. In order to support the draw of electrical energy during this final activity, a small electrical power subsystem can be provided on the lower (cargo) airship, powered by the array of solar cells which remain onboard.

In the event of a catastrophic system failure involving e.g. the total and unexpected loss of an upper airship in a tethered airship system, the tether can be severed at the lower (cargo) airship in order to prevent damage to the lower (cargo) airship by the tether (for example, through abrasion of the hull of the lower airship by the tether as it dangled below the lower airship for an extended period of time). The upper airship, along with the length of tether deployed at that time, will fall to Earth. If the lower airship along with the detachable payload return vehicle retains maneuvering capability, it can be maneuvered to a desired geographic location where the payload return vehicle can be detached and returned safely to Earth, and the lower (cargo) airship can be disposed-of as discussed herein. Alternatively, the lower (cargo) airship along with the payload return vehicle can be allowed to drift, and the payload return vehicle can be released and flown to a landing site of opportunity at a later time.

In the event of a catastrophic system failure involving e.g. the total and unexpected loss of the lifting capability of the lower (cargo) airship, the payload return vehicle can be immediately released and either flown to a suitable landing site (if one is available), or at least ditched in an area expected to be free of human activity.

The High-Altitude Airship in a Tethered Airship System

The high-altitude airship in a tethered airship system may undergo large changes in altitude as the system attempts to equalize the drag forces on the two airships. For example, in an equatorial mission intended to perform station-keeping using ambient winds, the high-altitude airship (the “upper airship”) might operate over an altitude range of 25-37 km (although other altitude ranges are feasible depending on the details of the system design). Over this altitude range, the volume of lifting gas (at roughly ambient pressure or slightly above) will vary by a ratio of 5:1. For example, if the lifting gas fills a volume of roughly 100,000 m³ at 25 km altitude, it will fill a volume of roughly 500,000 m³ at 37 km altitude. If the airship is designed to maintain a desired aerodynamic shape at its maximum altitude by virtue of holding the lifting gas at slightly greater than ambient pressure (for example, 100 Pa above ambient), this approach will not work at the lower altitude where the lifting gas, even without overpressure, fills no more than roughly 100,000 m³. If the shape is not maintained, baseline drag in wind may be adversely affected, it may be difficult or infeasible to adjust airship drag in order to assist in station-keeping, and associated airship structures (such as a parafoil and parachute, if present as part of the system) might not be properly supported or suspended (depending on the system design). Thus, it is desirable to maintain a proper aerodynamic shape throughout the normal altitude operating range of the airship.

Certain elements of one example of an upper airship are illustrated in a cartoon fashion in FIG. 8, showing both a side view and a front view of an upper stratospheric airship intended to operate as part of a tethered airship system. In the description of this example, numerical values generally follow other examples provided herein, but are only intended to illustrate the general elements of the inventive concept as gross vehicle weight will be affected by some of the accommodations discussed. Thus, in this example, an outer hull 800 is designed to withstand an internal pressure differential (relative to ambient) of 100 Pa. It provides a total internal volume of slightly more than 500,000 m³. The airship includes multiple internal ballonets intended to be filled with lifting gas, including bow ballonet 810, stern ballonet 820, three longitudinal ballonets 830 (illustrated), and central ballonet 840. In an example, all internal ballonets except the central ballonet 840 are designed to withstand the same pressure differential as the outer hull, and collectively include a lifting volume less than 100,000 m³. Thus, a non-zero volume of lifting gas may be stored in the central ballonet 840, at the lowest operating altitude of the upper airship, even when the other internal ballonets are pressurized with lifting gas to their maximum internal pressure differential. A pressurization subsystem (not shown), including one or more pumps and controllable valves, is adapted to move lifting gas among the various internal ballonets. Emergency pressure relief valves may also be provided to ensure that the internal ballonets are not destroyed by excess internal pressure. For example, emergency pressure relief valves may be adapted to vent lifting gas from the pressurized internal ballonets into central ballonet 840. In an example, the pressurized ballonets are bonded to the outer hull 800 in a nonzero number of places, in order to help maintain shape and provide support for the suspended tether and associated structures, and the central unpressurized ballonet is bonded to the inner surface of the upper outer hull in a nonzero number of places (it is allowed to inflate and expand downward through the central portion of the airship).

Bow ballonet 810 includes a central duct 805, along with a controllable aperture 807, which can be controlled to allow ambient air to move from outside the airship into the internal spaces of the airship (for example, inside the outer hull between the bow and stern ballonets, filling the empty space that is not filled by the central ballonet 840). Controllable pressure relief valves on the outer hull allow this internal air to be vented from the airship at will, or automatically in the event of an overpressure condition. In some examples, the controllable aperture 807 also includes a pump that can be used to pressurize the internal spaces of the airship (with ambient air) to pressures greater than the static pressure available from the ambient wind field.

Under normal operating conditions, the pressurized internal ballonets provide a semi-rigid structural framework that maintains the general outline of a desired aerodynamic shape of the upper airship. Furthermore, in a non-zero wind field, the static pressure experienced by the airship at its bow duct will allow a slight over-pressurization of the non-lifting volume of the airship, above ambient pressure, according (generally) to the equation ΔP=(1/2)ρv² where ΔP is the static pressure (the induced overpressure due to the ram force of the wind), ρ=ambient atmospheric density, and v=airspeed (wind speed). The controllable aperture 807 can be opened to allow this pressurization and closed to preserve this pressurization even when the winds abate. This will tend to maintain a desired aerodynamic shape for the airship as a whole. In an example where the controllable aperture 807 also includes a pump (and other examples where a pump is provided and adapted to pressurize the internal spaces of the airship), a greater internal pressure can be generated, which will tend to more precisely maintain a desired aerodynamic shape, and reduce the level of induced drag due to flutter of the hull surface.

It should be noted that an increase in internal pressurization also results in an increase in gross vehicle weight—the pressurized air acts as ballast due to its slightly higher density relative to the ambient air. However, in many circumstances the excess mass is small. For example, at 25 km altitude (ambient atmospheric pressure approximately 2511 Pa) and with 400,000 m³ of non-lifting volume, the excess mass is roughly 50 kg at 20 m/s airspeed (of course growing at greater airspeed).

With variable winds and e.g. a controllable aperture 807 along with controllable hull vents to allow the controllable release of internal air, an increased internal pressure can be captured during a high wind condition and maintained as the wind abates. Furthermore, if the upper airship is subsequently allowed to rise to a higher altitude, the internal air can be “bled off” in a controlled manner to reach and maintain a desired internal pressure above ambient. Furthermore, as an alternative to a pump or similar capability that can mechanically pressurize the hull with ambient air at a desired operating altitude of the upper airship, the upper airship can be temporarily “reeled down” to an altitude slightly below its currently-desired altitude, pressurized to ambient pressure or slightly above through action of the wind, and then allowed to ascend to its desired altitude with the controllable aperture 807 closed. This will provide additional pressurization of the upper airship.

Also shown in FIG. 8 are suspension lines 860 and adjustable control lines 862, supporting longitudinal beam 870 with its tether attachment point 890. The length of adjustable control lines 862 can be adjusted to control the angle of attack of the airship. In some examples, the attachment point of the tether 890 can be adjusted fore-and-aft along longitudinal beam 870.

In some examples, the upper airship of a tethered airship system is associated with a parafoil and/or parachute intended to provide lift, lateral steering forces, and adjustable drag. One or both of these structures (a parachute and/or parafoil) can be suspended from the stern of the airship in low-wind conditions (i.e., in order to facilitate proper opening), with control lines and tension lines attached to control subsystems spaced below the airship along the tether. In general, relative to a nominal low-drag flight condition, drag can be adjusted (if desired) by changing the angle of attack of the airship, adjusting an aperture of a parachute, or adjusting the angle of attack of a parafoil. Lift can be adjusted by changing the angle of attack of the airship or adjusting the angle of attack of a parafoil. Lateral steering forces can be changed by controlling the parafoil.

If the airship is also provisioned with an effective rudder, effective average drag can also be increased by commanding oscillatory yaw variations relative to the ambient wind field. The non-zero angles of side slip, combined with induced turbulent flow on both sides of the airship, will increase effective average drag. In an example, the yaw rate and period of oscillation is controlled to avoid potentially stressful, damaging or destructive oscillatory movements of the longitudinal beam 870.

An additional method to increase effective drag is to intentionally reduce the internal pressure of the non-lifting volume of the airship in order to induce fluttering (i.e., luffing or flapping) of the hull surface in the ambient wind.

These techniques (oscillatory yaw variations and induced fluttering of the hull surface) can be used in combination with, or in lieu of, a parachute and other methods of controlling drag.

The preceding description has been presented merely to illustrate and describe examples of the principles described. In some of the figures the relative dimensions of components have been altered for purposes of description. This description is not intended to be exhaustive or to limit these principles to any precise form disclosed. Many modifications and variations are possible in light of the above teaching. 

What is claimed is:
 1. A high-altitude airship system comprising a high-altitude airship, a cargo airship, and a tether connecting the high-altitude airship to the cargo airship, wherein the cargo airship comprises: a lifting gas reservoir; a releasable mass comprising an unmanned aerial vehicle with a payload adapted to perform at least one of: communications relay; signals intercept; remote surveillance; Earth observation; astronomical observation; or ballistic missile engagement; wherein the unmanned aerial vehicle comprises a propulsion system; and wherein orientation of the releasable mass is configured to be adjusted relative to the cargo airship and an external wind field.
 2. The high-altitude airship system of claim 1, wherein the high-altitude airship is adapted to inflate non-lifting volumes of the high-altitude airship with ambient air.
 3. A high-altitude airship system comprising a high-altitude airship, a cargo airship, and a tether connecting the high-altitude airship to the cargo airship, wherein the cargo airship comprises: a lifting gas reservoir; a first releasable mass comprising a pump to transfer lifting gas from the lifting gas reservoir to the high-altitude airship; a second releasable mass comprising an unmanned aerial vehicle with a payload adapted to perform at least one of: communications relay; signals intercept; remote surveillance; Earth observation; astronomical observation, or ballistic missile engagement; and wherein the unmanned aerial vehicle comprises a propulsion system.
 4. The high-altitude airship system of claim 2, wherein orientation of the releasable mass comprising an unmanned vehicle with a payload is configured to be adjusted relative to the cargo airship and an external wind field.
 5. The high-altitude airship system of claim 3, wherein the high-altitude airship is adapted to inflate non-lifting volumes of the high-altitude airship ‘with ambient air. 